The present invention generally relates to apparatus and methods for providing a cooling flow to a high pressure turbine rotor. More specifically, the present invention relates to apparatus and methods relating to providing a cooling flow to a turbine rotor by controlling the size and thus the spacing of cooling holes through a disk coverplate.
Turbine engines may include a compressor section, wherein inlet air is compressed, followed by a combustor section wherein fuel is combusted with the compressed air to generate exhaust gas. The exhaust gas is then directed to a turbine section, wherein energy is extracted from the exhaust gas.
The turbine section may comprise a rotor assembly. The rotor assembly may include a plurality of turbine blades installed on a rotatable disk. During operation, the turbine blades, the rotating disk, and other components of the turbine section may be exposed to elevated gas path temperatures, and thus may require cooling. Cooling may be provided to turbine section components using cooling air extracted from other parts of the engine. For example, cooling air may be supplied from the combustor plenum. Cooling air may be bled from the combustor plenum at compressor-discharge conditions and directed to the rotor assembly through a stationary Tangential OnBoard Injector (TOBI). The cooling air is directed by the TOBI at cooling holes disposed through a rotating disk coverplate, which is attached to the rotor assembly. The cooling holes serve as an inlet for cooling air, and are in fluid communication with the turbine blades. A portion of the cooling air leaving the TOBI traverses the distance between the TOBI exit and enters the cooling holes, which ultimately provides cooling to the turbine blades.
It is common in the art to provide a maximum number of cooling holes in the disk coverplate. The number of cooling holes in the disk coverplate must also be balanced with maintaining the physical integrity of the turbine rotor assembly. Typically, the distance between the cooling holes is less than or equal to twice the average diameter of two adjacent holes. This so called “50%” spacing allows for the maximum number of holes while maintaining adequate strength and integrity of the turbine rotor assembly. At less than full power however, these practices result in wasted energy and thus a diminution in overall engine efficiency.
To insure the turbine rotor assembly's mechanical integrity, it is also important to supply a portion of the cooling air to a purge cavity upstream of the rotor assembly. Thus, the cooling air which exits the TOBI but does not enter the cooling holes, is directed to a purged cavity which prevents hot gas-path flow ingestion into unwanted areas of the turbine section.
The cooling effectiveness of the blade is a strong function of the amount of cooling air and the pressure level at which the cooling air is supplied to the blade. Increased flow rates of cooling airflow combined with elevated coolant supply pressure can be used to improve blade cooling effectiveness. However, since cooling airflows do not contribute work to the turbine, excessive cooling air flow results in an undesirable increase in engine Specific Fuel Consumption (SFC) and reduces specific power of the gas turbine engine.
Typically, the turbine blades are only subjected to high gas temperatures and stresses at high engine power settings (i.e., full power), while at lower power settings, the blade does not need to be aggressively cooled. Hence, it would be beneficial to reduce the amount of cooling flow at less than full power settings, while still being able to maintain an adequate cooling air flow during full power settings.